I've had some bad experiences with this disease. And with other tick diseases- including Rocky Mountain Spotted Fever. Did your vet test for titer levels, or just a snap test that shows the dog has been exposed? A titer is more expensive, and doesn't always tell you how seriously ill the dog is. I'd recommend asking the vet about a PCR (polymerase chain reaction for you organic chemists out there) test instead. That test actually tests for the organism in the dog's blood. If I'd known what I know now, I'd always insist on a PCR test- not a titer, and not just a "positive" on a SNAP test.
Does your dog seem sick at all? When my dog had ehrlichia, he was just kind of "off"- weak, tired, coughing, not eating well. When I took him to my vet to get checked out he actually had heart arrhythmia, which can happen with this disease. I rushed him to a vet teaching hospital (I thought he'd die on the way there... it was not fun). Put him on docycycline hyclate and prednisone- pred was only for about 10 days or so.
I'd take this disease very seriously, but I'd want more testing. Based on my experience, and things I've read, I'd continue doxycycline at a fairly high dose for about 6 months. Doxy has been really expensive recently, if it still is, look into getting a prescription for a legitimate Canadian pharmacy.